This invention relates generally to security for cable television plant transmissions and particularly to an enhanced security system for a Zenith Z-TAC SSAVI (suppressed sync, audio, video inversion) scrambling system.
In early Z-TAC systems, a mode control signal in the form of a flag on a horizontal line in the vertical blanking interval (VBI) of the television signal indicates to the subscriber station decoder whether the following frame of video is to be inverted or not. A later version utilizes encrypted mode data on a horizontal line in the VBI (from which encrypted data is decoded by the decoder) for controlling the state of the subscriber station inverting amplifier. Security of a later version was improved because of the secrecy of the algorithm used for decoding of the mode data that controlled video inversion and the secrecy of the placement of a decoding key in the VBI data.
A still later version of the Z-TAC system employed an extension of in-band data in the VBI and an out-of-band data channel for extended features such as enhanced program tag capability, an electronic program guide, various dealer services, on screen display menus and the like. The out-of-band data channel also enables communications with subscriber station decoders in the cable system that are not tuned to a particular channel. The present invention utilizes the extended in-band data and the out-of-band data channel to further enhance the security of the video inversion of the Z-TAC system.